Compensated power supply amplifier



April 18, 1950 GRANT 2,504,829

COMPENSATED POWER SUPPLY AMPLIFIER Filed Feb. 1, 1946 WITNESSES: INVENTOR fz/yem? fi'Gr'anz.

f/XM @VW BY W,

Patented Apr. 18, 1 950 AMPLIF IER Eugene F. Grant, I-lempstead, N. Y., assignor to.

Westinghouse Electric Cprporation, East Pittsburgh,..Pa;, a-corporation of, Pennsylvania:

. Application February 1, 1946, Se1:ial'No. 644,806

I 1 Claim;

My---invention' relates to electronic amplifiers, and; in particular, relates to a compensating grid-bias system forsuch amplifiers.

In high vacuum electron-tubes embodying con trol grids, the plate current which flows when a:

certainbias voltage is" impressed on the, control grid is supposed to have a certain prescribed value for a given type or style-number of tube. However, in' actual practice, the plate-current varies" in difierent samples of supposedly; identical tubes; furthermore, itmay vary alsoin-response to variation of the environment temperature about the tube; or inresponse to. variation'in the direct-current voltage source supplying the plate current. Under the limitations of'structureimposed by certain fieldsof useyin' some types'of modern vacuum tubes, these unwantedvariations' in plate current are highly undesirable-factors in their operation. In many instances,- these variations can be largelyminimizedby providing a cathode resistor in seriesbetween the cathode. of the tube and the commonterminal ofthe plate and grid circuits, such common terminal being,

in ordinary practice, connected to ground; This cathode resistor is traversediby the plate cure rent and any deviation in the latter from the V prescribed value impresses a stronger negative bias on the grid, thereby tending to minimize the current change. However, in certain types of systems in which it is desiredlto use electron am plifier tubes, notably circuits involving'extra high frequencies, serious disadvantages have. been} found to arise from the employment of .such catlie ode resistors. employed, it is possible .to vby-passthecathode resistor with a capacitor, thereby. largely preventing the cathode resistor from prodi cingree generative or degenerative efiects in the highfrequency currents; but in the case of some of the ultra-high frequencies, difficulty arises in thusby-passing the cathode resistor because the physical length of the condenser and its leads is great enough to produce a substantial inductive reactance and because size of the direct current requiredifrom the tube results .inan undesirablylow impedance of the cathode. circuit.

One object of. my invention isnaccordinglytto.

Where ordinary frequencies. are.

2 tubes in which the cathodeofi the tube can be connected directly togroundr Still another object of my invention is to provide a circuit for operating electron amplifier tubes which avoids the use of a cathode resistor for compensating the tendency of the static plate current to vary from a desired value, but-which effects an equivalent compensationagainst the tendency of such plate current variation to occur;

Other objects of my invention will become apparent upon reading the following description, taken in connection with thedrawing, in which, the single figure is a schematic diagram of one circuit embodying the principles ofmy invention.

Referring in detail toFigure l of the drawing, an electron tube 1, preferably of the high vac uum type, is provided with a cathode 2, a control grid 3 and may be provided in. additicn with a screen grid 4 and a suppressor: grid 5 of conventional type well known in the art. However, it is to be understood: that my, invention is entirely. applicable to tubesin which the screen; grid 4 and suppressor grid 5 are omitted. The cathode" 2 is connected 1 directly toagroundgandgthence to the negative terminal of a voltagesource, 6 haw: ing'its positive'terminal'connected through a re-.. sistor l and anzinductor 8rto-the anode 9 .of therof input signals having itsother terminal cone,

nected to the inputof the. cathode 2'. The con-' trol grid 3 is likewise connected through a,ref sistor'll and another resistor I8 tothe negative terminal of a biastvoltage source l91having its positive terminal'connected to the cathode, for example, by way of the ground thereon. ,The

common terminal of the resistors I! and i8 is, connected to the cathode 2" through a bypass condenser 2| and is also connected to the common terminal of the resistor I and the inductor 8 by a resistor 22. R7 is preferably small compared to R18 and R22.

The mode of operation in effecting compensation of the above-described circuit will now be pointed out. If it be supposed that for any reason a small voltage increase a is impressed on the control grid 3, an increase b=Ga would normally occur in the plate current were the plate voltage held constant. Here Gis the mutual conductance of the tube. Since this increase occurs in the plate current flowing from th source 6 to the anode 9 through resistor I, the potential difference acting to send current through the resistor 22, and the resistor I8 will be decreased by an amount equal to bR-z, where R1 is the ohmic resistance of resistor 1. This will result in a deisequalto where R18 is the resistance of resistor 18 and R22- is th resistance of the resistor 22. The voltage drop through the resistor I8 is impressed between the control grid 3 and the grounded cathode 2, and in accordance with the well known theory of operation .of electron tubes, this will produce a decrease in the plate current flowing from the anode 9, which is .equalto I G bR1R18 v I l8+ 22 I The total change in plate current in the tube is thus the sum of theabove-mentioned increase and decrease; i. e., it is G 7 lS I l8+ 22 I But this plate current change is also b; hence and so I 7 Ga In contrast to this, a simple three-electrode tube in which the change B in current in the plate circuit produces no change in voltage on the control electrode, Which might be termed. a tube operated without compensation, would obey the law B=Ga. It .is thus evident that the change 17 in plate'curren't with compensation is'f related to the changeB in plate current without compensation by'the following relationship:

is the direct-current gain of the amplifier circuit causing the loop embodying the control grid, the cathode and the .anodeelectrode. It is, in general, desirable that this gain be made as large as possible with a given desired total variation.

As is shown by many writers on electron tubes,

the variation in the plate current with self-bias of the type shown in Fig. 2 is equal to '10 crease in voltage drop across the resistor iL whic h I I I the same tube without a cathode resistor.

l+GR, I I times the variation in plate current occurring iii here is the resistance of the cathode resistor.- Comparing this figure with the directqcurrent gain of the circuit I have. described, it will be evident that the two-systems will be equivalent in compensating efiect if It is thus evident that the circuit I have described can be so proportioned so as to have a compensating eifect equivalent to that attained by the use of cathode resistors in electronic amplifier tubes. 7

While I have mentioned that the arrangement is useful in the ultra-high frequency field, it

has advantages under lcertain conditions for audio-frequency amplifiers. In such case it may b desirable to omitcapacitors H and I2 and inductor 8.

In Fig. 2 I have shownia conventional cathoderesistor circuit for an; amplifier comprising a tube 23 having i ts catho de grounded through a resistor R0 shunted by a; capacitor 24. The input circuit is connected between ground and the In combination with an electron tube having a control electrode, a cathode and an anode, a

direct-current source-having its negative terminal, connected tosaidcathode and its positive ter' minal connected through.a,firstresistor to said anode, a second 'direct current source havingits positive terminal connected to said cathodeand,

having its negative terminal connected through resistance means to said control electrode, a second jresistor interconnecting an intermediate point on said resistance means withthe terminal of said first resistor which is the more remote from said cathode in the circuit set forth above,

" and a pair of serially connected capacitors shunting said second resistor andhaving their common terminal connected to said cathode.

IEUGIENE r. GRANT. I

I REFERENCES CITED The following, references are. of record in the file of this patent}-..

UNITED STATES PATENTS Number Name" Date 2,202,522 Gloes s May 28, 1940 2,229,702

Larsen Jan. 28, 1941 

